The control of aircraft on the ground is a complex task. Airfields generally provide control and guidance to aircraft while taxiing or standing both before takeoff and after landing.
Airfield lighting control systems have been developed to avoid incursions onto an active runway or taxiway, thereby reducing the likelihood of collisions between aircraft, especially during conditions of low visibility.
At airports it is important to provide communication between a computer in the tower and individual lighting control elements on the airfield. This is desirable to monitor and control a variety of conditions, for example, to determine the location of lamps which have failed, the status of stop bar lights, and to implement surface movement guidance and control systems.
At least one system has been provided for switching and monitoring individual lights on the airfield. This airfield lighting system is disclosed in the patent to Runyon et al. (U.S. Pat. No. 5,485,151). This system, however, attaches directly to the high voltage primary on the series circuit, which may tend to increase its size and cost.
The lamps in an airfield lighting system are typically powered by current isolation transformers having primary windings that are connected in series and secondary windings that are coupled to provide power to the lamps. Because the transformers are connected in series, the failure of a single lamp will not affect the operation of the remaining lamps in the circuit.
The circuit is powered by a device called a constant current regulator (CCR). A CCR may provide a regulated current in either three or five discrete steps. The maximum output current may be as low as 6.6 A or as high as 20 A. The current is changed to control the brightness of the lamps to compensate for different weather or visibility conditions.
Airfield lighting systems need to be carefully monitored to ensure that failed lamps are promptly discovered and that remedial action is taken as soon as possible. One type of lamp-out monitoring system operates by monitoring a parameter which is only available at the output of the CCR, such as voltage, current, or waveshape deviation. A disadvantage of this type of system is that the location of the failed lamp may be difficult to determine.
Another type of monitoring system uses an electronic circuit coupled to the secondary winding of the current isolation transformer. When a lamp fails, the circuit periodically shorts and opens the secondary during a predefined time period. The failed lamp location is identified by a master unit in a vault computer which detects a change in impedance during the predefined time period. A disadvantage of this system is that communication is uni-directional. Commands may not be sent to devices connected to the secondary of the isolation transformers.
Another type of system also uses an electronic circuit on the current transformer secondary and communication is accomplished by varying the impedance on the secondary. The impedance variation is controlled according to a predefined communication protocol. The master unit in the vault computer detects the information according to the protocol. The master can also send out messages, using its own protocol, to the circuits on the series circuit. In this manner, monitoring and control information can be sent back and forth on the series circuit. A disadvantage of this system is that a relatively low frequency signal may be needed to provide sufficient signal amplitude through the current transformer for the signal to be detected by the vault computer. Another disadvantage relates to the use of impedance changes to provide communication. If there is a long wire connecting the transformer secondary to the load, the impedance of this secondary wire may reduce the amplitude of the transmitted impedance changes. A system of this type may also be susceptible to radio-frequency interference (RFI) and to overall impedance changes in the series circuit, such as when the circuit is physically reconfigured or when lamps are added to the circuit.
Another type of system electrically connects to both the primary and secondary of the current transformer. Bi-directional communication with the master is achieved by directly transmitting a single base high frequency on the series circuit. This configuration allows a relatively high communication rate with a relatively large amount of data within the protocol. A disadvantage of this type of system is that the connection to the primary may be subject to high voltages and may result in higher cost compared to systems that connect only to the secondary.